The present invention relates to land-based or industrial gas turbines, for example, for electrical power generation, and particularly to the mechanical nozzle airfoil preloading device.
Low cycle fatigue (LCF) is one of the major life-limiting degradation modes in advanced industrial gas turbine nozzles. It is caused by cyclic, thermal and mechanical loads associated with gas turbine start-up, operation, and shutdown cycles. The effects of cyclic modes on LCF life generally vary within a xe2x80x9cstrain A-ratio,xe2x80x9d or the ratio of alternating to mean strain, among other things. For a given level of cyclic load, the most damaging LCF cycle is usually one involving a hold period in compression, commonly known as LCF strain A-ratio of xe2x88x921. By contrast, the least damaging LCF cycle is the one involving a hold period at zero strain, or LCF strain A-ratio of +1. The problem is that the prevailing LCF conditions for a nozzle at LCF life-limiting locations are usually a low life causing strain A-ratio of xe2x88x921.
In the past, LCF life improvements for a nozzle have been sought by traditional approaches such as a design optimization to reduce LCF stresses and temperatures, and new material selections with improved LCF capabilities. With a recent gas turbine industry wide trend of increasing firing temperatures and more efficient nozzle cooling schemes, however, nozzle design stresses and temperatures often exceed the limits of even the strongest materials currently available.
This invention addresses the LCF life problem by pre-straining a nozzle such that the strain A-ratios at the life critical locations will be shifted from xe2x88x921 to +1, resulting in a higher LCF life resulting. In the exemplary embodiment, an OEM installable mechanical device is designed to pre-strain a nozzle to counter the LCF loads, thereby extending its service life beyond the usual material limits of the conventional nozzle. More specifically, a preloading rod is inserted through each vane or airfoil of the nozzle, and fixed at one end, preferably the radial inner end. The pre-loading device, which may be in the form of a threaded nut engaging an exteriorly threaded surface of the rod, is tightened down on the rod, externally of the nozzle cover, thereby placing the airfoil in compression. After the nut has been tightened to achieve the desired pre-load, the rod may be welded to the radially outer cover of the nozzle, thereby fixing the pre-load. Preferably, the rod is located along the leading edge of the airfoil, since this is the most life-critical location in the airfoil. If considered advantageous, however, additional rods may be added in other locations within the airfoil.
Accordingly, the present invention relates to a method of increasing low cycle fatigue life of a turbine nozzle having a plurality of stationary airfoils extending between radially inner and outer ring segments comprising a) providing at least one radial passage in each of the plurality of airfoils; b) installing a rod in the radial passage extending between the inner and outer ring segments and fixing one end of the rod to one of the inner and outer rings; and c) pre-loading the rod to compress the airfoil between the inner and outer ring segments.
The invention also relates to a nozzle for a gas turbine comprising a plurality of airfoils extending between radially inner and outer ring segments; each airfoil having means for pre-loading the airfoil in compression.